Musannaf Ibn Abi Shayba 1:1111

Abu Mu'awiyah narrated to us, from al-Shaybani, from Sa'id ibn Jubayr, who said: It would happen that [a verse] was revealed while I was on a journey to relieve myself of excrement or urine, and I would not catch up with my companions until I had recited a portion of the Qur'an before I performed ablution.